WASHINGTON – In an abrupt move to keep the GOP tax bill on track, the Senate on Tuesday stripped part of a tax break proposed by Texas Republican Ted Cruz that had been touted as a "game changer" for families who home-school their children.

Cruz's "Student Opportunity Amendment," decried by a top Democratic foe as "a backdoor assault on the public K-12 education system," fell victim to a budget rule protecting the tax legislation from a Democratic filibuster.

But only in part.

That was the part of Cruz's measure allowing families to use tax-advantaged 529 accounts for home schooling expenses, which Senate aides described as a small but still important piece of his legislation.

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It was one of three pieces of the $1.5 trillion Republican tax legislation that came under a late parliamentary challenge to ensure passage without Democratic votes.

The Senate move came hours after the House passed the GOP tax bill 227-203, a milestone that was supposed to set up Senate passage later in the day. But the need to amend the Senate language forced House leaders to schedule another vote on Wednesday for final passage of a bill with identical language for President Donald Trump's signature.

Another part of the Cruz amendment allowing families to use 529 accounts for private and religious schooling apparently would not be affected. The full tax subsidy also could still apply in states that define home-schooling as private education.

States like Texas, however, do not define home schools as private schools, meaning the tax benefit would not be available for home schoolers in the Lone Star State unless there was a change in state law.

Some observers say that the distinction is likely to revive the debate within the home-schooling movement as to whether to seek IRS recognition – and possibly the federal regulation that goes with it.

The problem with the Cruz measure came to light after a review by the Senate parliamentarian found three items that violated Senate rules that govern the Republicans' fast-track process for avoiding a filibuster.

Facing unified Democratic opposition to the tax legislation, the 52-member Republican majority in the Senate would not have the votes to break a filibuster, which requires 60 votes to overcome.

Amid last-minute negotiations, one interpretation of the parliamentarian's ruling was that since federal law tax does not currently address home-schooling, any measure providing a tax break for home-schooling families would fall outside the paraments of the fast track process known as "reconciliation."

In a floor speech late Tuesday night, Cruz took issue with the parliamentary ruling, calling it "contrary to federal law."

Some 1.8 million kids are home-schooled in America, according to estimates. Cruz called the Democrats' objections "mean-spirited (and) vindictive."

In recent weeks, Cruz's 2018 Senate campaign has been heralding his school choice measure to religious conservatives, describing it as a turning point in the conservative struggle to allow public funds to be steered toward home-schoolers or private school students.

In an email to supporters last week the Cruz campaign highlighted a recent opinion piece by Drew White, a senior policy analyst for the conservative Texas Public Policy Foundation, calling the Cruz measure a "game changer."

The original amendment -- passed after Vice President Mike Pence broke a 50-50 tie -- would allow $10,000 per child to be taken from tax-privileged 529 savings plans each year. Much like Roth IRAs, the 529 plans use after-tax contributions to accumulate and grow tax free.

Under the proposed change, Cruz's amendment would still expand the popular 529 college savings plans to include K-12 elementary and secondary tuition, but not for the educational expenses for home-schooled children.

According to Congress' Joint Committee on Taxation, the cost before the restriction on home-schooling expenses would cost the Treasury about $500 million over 10 years. Much of that was estimated to be taken up by the cost of private school tuition, not home schooling expenses. Home schoolers, however, represent a significant portion of the evangelical and Christian conservative base that has backed Cruz in his runs for the Senate and the White House.

Democrats have criticized Cruz's amendment as a boon to wealthy families who can afford to send their kids to private school. But the parliamentary challenge is limited to home-schoolers.

The Democratic objection was raised by Sen. Bernie Sanders, I-Vt., ranking member of the Budget Committee, and Sen. Ron Wyden, D-Ore., ranking member of the Finance Committee.

"In the mad dash to provide tax breaks for their billionaire campaign contributors, our Republican colleagues forgot to comply with the rules of the Senate," they said in a joint statement. "Instead of providing tax breaks to the wealthiest people and most profitable corporations, we need to rebuild the disappearing middle class."

In his bid for the White House last year, Cruz often called school choice "the civil rights issue of our time."